The opioid epidemic is one of the largest public health crises in modern history. First-responders, emergency room personnel, and overdose observation units encounter drug overdose patients under varying degrees of sedation and physiological distress. The time course of drug effects, and the dosing requirements of reversal medications, can be difficult if not impossible to gauge, given the different potencies of opioids that could be in effect, and the unknown drug sensitivity of individual drug users. The goal of this project is to characterize the EEG signatures of fentanyl in humans, and their relationship with states of unconsciousness and systemic physiology. We propose to do so within the highly controlled confines of the operating room, where fentanyl is routinely administered general anesthesia. In doing so, we aim to construct a clinical and quantitative neurophysiological basis for fentanyl's effects that can be used to develop new monitors to improve care of opioid overdose patients.